The in-band on-channel digital radio broadcasting standard for the Frequency Modulation (FM) band is defined by the FM-part of the NRSC-5 standard “National Radio Systems Committee (NRSC) NRSC-5-C, “In-band/on-channel Digital Radio Broadcasting Standard”, September, 2011”. This standard is referred to below as REF [1]. Moreover, REF [1] is the basis for the transmitted IBOC signals of the digital corporation iBiquity™ that can be received by an HDradio certified receiver.
The basics of this standard are discussed below, based on some of the corresponding documents belonging to REF [1] and specifically document; “HD Radio™ Air Interface Design Description Layer 1 FM”, Doc. No.: SY_IDD_1011sG Rev. G, Aug. 23, 2011. This document is referred to below as REF [2].
One type of IBOC signal is the so-called “Hybrid IBOC FM” signal, which is denoted “Hybrid IBOC” in this document.
FIG. 1 shows this hybrid signal in simplified form. The hybrid signal is a combination/addition of an analog FM-signal and a digitally-modulated signal. The analog FM-signal 10 occupies a bandwidth of 200 kHz, i.e., between −100 kHz and +100 kHz separated from the carrier-frequency. The digitally-modulated signal occupies a bandwidth of roughly 200 kHz. However, the digitally-modulated signal is separated into a lower-sideband 12 and an upper-sideband 14 both with a bandwidth of roughly 100 kHz. The lower-sideband is spectrally positioned at a distance of 100 kHz below the carrier-frequency. The upper side-band is spectrally positioned at a distance of 100 kHz above the carrier-frequency.
Each digital sideband comprises a set of a maximum of 14 partitions depending on the service mode. Each partition has 19 subcarriers. Depending on the service mode the number of partitions is 10, 11, 12, or 14.
The total power of the digitally-modulated signal is approximately a factor of hundred smaller than the power of the analog host-FM-signal. The hybrid IBOC signal can be seen as a sort of noisy FM signal.
In addition, the digitally-modulated signal uses OFDM, where the number of subcarriers can vary depending on the selected service/transmission-mode. The so-called “channel-grid” (the reserved channel-bandwidth for an analog FM-signal) is 200 kHz. As a consequence, the lower and upper digital OFDM-sidebands are using the 1st adjacent lower, and upper neighboring FM-channels.
Another type of IBOC signal is an all-digital implementation. For the all-digital IBOC FM signal, the analog FM-signal is replaced by a (secondary) digitally-modulated signal. In the all-digital mode the bandwidth of the primary digital sidebands is fully expanded with lower-power secondary sidebands. In particular the number of OFDM subcarriers is increased to 1093.
FIG. 2 shows the spectrum plot of the all-digital IBOC signal. This is denoted “all-digital IBOC” in this document. The term (H)IBOC is used to refer to a system compatible with both signals.
The all-digital IBOC signal has a bandwidth of roughly 400 kHz, where also in the all-digital mode approximately 100 kHz of the lower and upper adjacent channels is occupied (outside the 200 kHz “channel-grid”).
The lower digital sideband is shown as 20 and the upper digital sideband is shown as 22. Each has a primary section 20a, 22a and a secondary section 20b, 22b. Each primary section and each secondary section is formed of 10 main (M) frequency partitions and 4 extended (E) frequency partitions each of 19 subcarriers. The secondary partitions 20b, 22b also have a protected partition (P) each of 12 subcarriers.
Note that the digital parts 12, 14 in FIG. 1 take the form of the primary sidebands 20a, 22a as shown in more detail in FIG. 2. However, in the hybrid-mode the number of extended frequency partitions can be; 0, 1, 2, or 4 depending on the transmitted service mode.
The table below is the table in Section 3.5 of REF [2] that defines the FM-system parameters for the OFDM-symbols, the OFDM-frames, and the OFDM-blocks.
Computed ValueParameter NameSymbolUnitsExact Value(To 4 significant figures)OFDM Subcarrier ΔfHz1488375/4096363.4 SpacingCyclic Prefix Widthαnone7/1285.469 × 10−2OFDM Symbol Tss(1 + α)/Δf =2.902 × 10−3Duration(135/128) · (4096/1488375)OFDM Symbol RateRsHz= 1/Ts344.5 L1 Frame DurationTfs65536/44100 = 512 · Ts 1.486L1 Frame RateRfHz= 1/Tf6.729 × 10−1L1 Block DurationTbs= 32 · Ts9.288 × 10−2L1 Block RateRbHz= 1/Tb10.77L1 Block Pair DurationTps= 64 · Ts1.858 × 10−1L1 Block Pair RateRpHz= 1/Tp 5.383Digital Diversity DelayNddnone3 = number of L1 frames3  Framesof diversity delayDigital Diversity DelayTdds= Ndd · Tf 4.458TimeAnalog Diversity DelayTdds= 3.0 · Tf 4.458Time
As can be seen from this table the OFDM-subcarrier spacing is defined as:
      Δ    ⁢                  ⁢    f    ⁢      =    def    ⁢            1488375      4096        ≈          363.4      ⁢                          ⁢              Hz        .            
The appropriate sampling-frequency for the OFDM-symbols as a function of the FFT-length can be written as, Fc=N*Δf, where N is the FFT-length. The total number of OFDM-subcarriers in the all-digital case is 2*546±1=1093. Note that in the hybrid mode (FIG. 1) only, at most 2*267 of active OFDM-carries are used/available. To perform efficient FFT and IFFT operations a radix-2 kind of FFT is preferable, which means that the (I)FFT length needs to be:N=2n≧1093→2048=211≧1093→N=2048.
The appropriate sampling-frequency becomes now:
      F    c    =            2048      *              1488375        4096              =                  1488375        2            ≈              744        ⁢                                  ⁢                  kHz          .                    
In addition, in Chapter 5, Section 5.1.1.1 Table 1 of REF [1] the definition of an Audio-frame is defined as:
“The unit of information payload exchanged from the audio interface and the audio codec protocol layer. Audio frames are comprised of 2048 audio samples at a sampling rate of 44.1 kHz.”
With reference to the signal-processing sampling-frequency, it is related, i.e., fixed to the audio sampling-frequency by:
      F    c    =                              135          4                *        44.1        ⁢        e        ⁢                                  ⁢        3            2        =                  135        8            *      44.1      ⁢      e      ⁢                          ⁢      3      
Such a fixed relation between the signal-processing and audio sampling-frequency can be especially desirable for maintaining stable decompressed audio quality.
Furthermore, Appendix-A of iBiquity™ Digital Corporation, “Transmission Signal Quality Metrics for FM IBOC signals”, Doc.No.:SY_TN_2646S Rev. 02, Aug. 24, 2011 provides suggestions for synchronization to the carrier frequency and to determine symbol timing. It shows that the signal processing for the “wide-band” approach operates with an (radix-2) FFT size of 2048 points on the corresponding clock speed of:
      F    c    =            1488375      2        =                            135          8                *        44.1        ⁢        e        ⁢                                  ⁢        3            =      744187.5      
Thus, it can be concluded that for the conventional “wide-band” (H)IBOC FM-receiver a 2048 points FFT at a sampling-frequency of ≈744 kHz is applied.
The digitally-modulated OFDM-signal only occupies roughly 200 kHz or 400 kHz with a maximum of 534, or 1093 subcarriers respectively, so that a sampling-frequency of ≈744 kHz and a 2048 points FFT is rather large, i.e., “wide-band”. Consequently, with the sampling-frequency or clock-speed of Fc≈744 kHz and the FFT-size of 2048, the “wide-band” conventional (H)IBOC approach, e.g., suggested by iBiquity (Trade Mark), will process the relatively small digitally-modulated OFDM spectrum with a processing-bandwidth (i.e., fundamental-interval) of:
      B    w    =            ±              1488375        4              ≈                  ±        372            ⁢                          ⁢      kHz      
Even in the all-digital case the all-digital IBOC FM-spectrum will not exceed a bandwidth of Bw≈±200 kHz, as can also be seen from FIG. 2. Thus, according to the Nyquist sampling-theorem, a sampling-frequency of Fc≧400 kHz should be sufficient to process and be able to fully recover the digitally-modulated OFDM-signals.
These observations form the basis of this invention, which aims to provide a “narrow-band” alternative for the conventional “wide-band” (H)IBOC FM-receiver.